Typically, a fuel battery has a cell stack formed by a number of power generating cells stacked together. With reference to FIGS. 9 to 12, a prior art power generating cell will be described. As shown in FIG. 9, a pair of upper and lower frames 13, 14 are connected to each other, and an electrode structure 15 is installed at the joint portion of the frames 13, 14. The electrode structure 15 is formed by a solid electrolyte membrane 16, an electrode catalyst layer 17 on the anode side, and an electrode catalyst layer 18 on the cathode side. The outer periphery of the solid electrolyte membrane 16 is held between the frames 13, 14. The anode-side electrode catalyst layer 17 is laid on the upper surface of the solid electrolyte membrane 16, and the cathode-side electrode catalyst layer 18 is laid on the lower surface of the solid electrolyte membrane 16. A first gas diffusion layer 19 is laid on the upper surface of the electrode catalyst layer 17, and a second gas diffusion layer 20 is laid on the lower surface of the electrode catalyst layer 18. Further, a first gas passage forming member 21 is laid on the upper surface of the first gas diffusion layer 19, and a second gas passage forming member 22 is laid on the lower surface of the second gas diffusion layer 20. A flat plate-like separator 23 is joined to the upper surface of the first gas passage forming member 21, and a flat plate-like separator 24 is joined to the lower surface of the second gas passage forming member 22.
The solid electrolyte membrane 16 is formed of a fluoropolymer film. As shown in FIG. 10, the electrode catalyst layer 17, 18 each have carbon particles 31 of diameters of several micrometers, and a great number of platinum (Pt) catalyst particles 32 adhere to the surface of each carbon particle 31. The catalyst particles 32 have a diameter of 2 nm. When electricity is generated by the fuel battery, the catalyst particles 32 function as catalyst that increases the power generation efficiency. The gas diffusion layers 19, 20 are formed of carbon paper.
FIG. 11 is an enlarged perspective view showing a part of the first and second gas passage forming members 21, 22. As shown in FIG. 11, the gas passage forming member 21 (22) is made of a metal lath plate, which has a great number of hexagonal ring portions 21a (22a) arranged alternately. Each ring portion 21a (22a) has a through hole 21b (22b). Fuel gas (oxidation gas) flows through gas passages formed by the ring portions 21a (22a) and the through holes 21b (22b).
As shown in FIG. 9, the frames 13, 14 form a supply passage M1 and a discharge passage M2 for fuel gas. The fuel gas supply passage M1 is used for supplying hydrogen gas, which serves as fuel gas, to the gas passages of the first gas passage forming member 21. The fuel gas discharge passage M2 is used for discharging fuel gas that has passed through the gas passages of the first gas passage forming member 21, or fuel off-gas, to the outside. Also, the frames 13, 14 form a supply passage and a discharge passage for oxidation gas. The oxidation gas supply passage is located at a position corresponding to the back side of the sheet of FIG. 9, and is used for supplying air serving as oxidation gas to the gas passages of the second gas passage forming member 22. The oxidation gas discharge passage is located at a position corresponding to the front side of the sheet of FIG. 9, and is used for discharging oxidation gas that has passed through the gas passages of the second gas passage forming member 22, or oxidation off-gas, to the outside.
Hydrogen gas from a hydrogen gas supply source (not shown) is supplied to the first gas passage forming member 21 through the supply passage M1 as shown by arrow P of FIG. 9, and air is supplied to the second gas passage forming member 22 from an air supply source (not shown). Accordingly, electricity is generated through an electrochemical reaction in the power generating cell. Specifically, hydrogen gas (H2) supplied to the first gas passage forming member 21 flows into the electrode catalyst layer 17 through the first gas diffusion layer 19. In the electrode catalyst layer 17, hydrogen (H2) is broken down to hydrogen ions (H+) and electrons (e−) as shown by chemical formula (1), and the potential of the electrode catalyst layer 17 becomes zero volts, or standard electrode potential, as known in the art.H2→2H++2e−  (1)
Hydrogen ions (H+) obtained through the above reaction reaches the cathode-side electrode catalyst layer 18 from the anode-side electrode catalyst layer 17 through the solid electrolyte membrane 16. Oxygen (O2) in the air supplied to the electrode catalyst layer 18 from the second gas passage forming member 22 chemically reacts with the hydrogen ions (H+) and the electrons (e−), which generates water as shown by the formula (2). Through the chemical reaction, the potential of the electrode catalyst layer 18 becomes approximately 1.0 volt, or standard electrode potential, as known in the art.½.O2+2H++2e−→H2O  (2)
In a normal power generation condition of the fuel battery, the potential of the anode-side electrode catalyst layer 17 (the first gas diffusion layer 19) is lower than the potential of the cathode-side electrode catalyst layer 18 (the second gas diffusion layer 20). Thus, compared to the second gas passage forming member 22, the first gas passage forming member 21 is less susceptible to metallic oxidation due to a high potential. Therefore, as shown in FIG. 12, an inexpensive stainless steel such as ferrite-based SUS having a low corrosion resistance. On the other hand, the second gas passage forming member 22, the potential of which can become high, is formed by a metal having a high corrosion resistance such as gold as shown in FIG. 12. Patent Document 1 discloses a power generating cell for a fuel battery having a similar structure to the structure shown in FIG. 9.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2007-87768